The present invention relates to an improved method and apparatus for use in casting an airfoil. The airfoil may have either a columnar grain or single crystal crystallographic structure.
A known apparatus for use in casting an airfoil with a columnar grain crystallographic structure is disclosed in U.S. Pat. No. 3,485,291. A known apparatus for casting an airfoil with a single crystal crystallographic structure is disclosed in U.S. Pat. No. 5,062,468. The airfoil molds disclosed in the aforementioned patents are of the top fill type. However, a mold of the bottom fill type is disclosed in U.S. Pat. No. 3,915,761.
During the casting of airfoils with the molds disclosed in the aforementioned patents, impurities tend to accumulate adjacent to a chill plate. As the molten metal in the mold solidifies, these impurities tend to migrate upward into the airfoil mold cavity. The upward migration of the impurities into the airfoil mold cavity may result in the formation of inclusions in the airfoil. Of course, the presence of inclusions is detrimental to the properties of the airfoil.
Airfoils having a columnar grain crystallographic structure have improved transverse properties if there are a relatively large number of columnar grains in the airfoil. Thus, if a portion of the airfoil is formed with a large number of columnar grains, the transverse properties of that portion of the airfoil are enhanced. Known airfoils having a fine columnar grain root portion may have as many as 100 columnar grains per square inch of cross sectional area.